A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A typical disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute (RPM).
Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a number of sectors, with each sector comprising a number of information fields, including fields for storing data, and sector identification and synchronization information, for example.
An actuator assembly typically includes a plurality of outwardly extending arms with one or more transducers and slider bodies being mounted on flexible suspensions. The slider body lifts the transducer head off the surface of the disk as the rate of spindle motor rotation increases, and causes the head to hover above the disk on an air bearing produced by high speed disk rotation. The distance between the head and the disk surface, which is typically on the order of 40-100 nanometers (nm), is commonly referred to as head-to-disk clearance or spacing.
Writing data to a magnetic data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer assembly sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals, commonly referred to as readback signals, in the read element.
Conventional data storage systems generally employ a closed-loop servo control system for positioning the read/write transducers to specified storage locations on the data storage disk. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read information for the purpose of following a specified track (i.e., track following) and locating (i.e., seeking) specified track and data sector locations on the disk.
In accordance with one known servo technique, embedded servo pattern information is written to the disk along segments extending in a direction generally outward from the center of the disk. The embedded servo patterns are thus formed between the data storing sectors of each track. It is noted that a servo sector typically contains a pattern of data, often termed a servo burst pattern, used to maintain optimum alignment of the read/write transducers over the centerline of a track when transferring data to and from specified data sectors on the track. The servo information may also include sector and track identification codes which are used to identify the location of the transducer.
Within the data storage system manufacturing industry, much attention is presently being focused on reducing head-to-disk clearance as part of the effort to increase the storage capacity of data storage disks. It is generally desirable to reduce the head-to-disk clearance in order to increase the readback signal sensitivity of the transducer to typically weaker magnetic transitions associated with higher density disks. When decreasing the transducer-to-disk clearance, however, the probability of detrimental contact between the sensitive transducer and an obstruction on the disk surface significantly increases.
A prevalent surface irregularity that afflicts an appreciable percentage of conventional data storage disks is generally referred to as an asperity. Asperities are isolated submicron-sized particles, typically comprising silicon carbide material, that are embedded in the disk substrate. No single mechanism has yet been identified as the source of such asperities, and it is believed that asperity defects arise from numerous sources. Such asperities are often large enough to interfere with the flight path of a typical slider/transducer assembly by impacting with the slider/transducer assembly at a very high velocity.
Further, asperities arising from the surface of a data storage disk are generally distributed in a highly random manner, and change in shape and size in response to changes in disk and ambient temperatures. A collision between a slider/transducer assembly and an asperity often renders the location of the asperity unusable for purposes of reading and writing information. Moreover, repeated contact between the slider/transducer assembly and asperity may cause damage of varying severity to the slider/transducer assembly.
Magneto-resistive (MR) transducers, for example, are particularly susceptible to interference from contact with asperities. It is well-known that MR transducers are very sensitive to variations in temperature, and are frequently used as temperature sensors in other applications. A collision between an MR transducer element and an asperity results in the production of heat, and a corresponding rise in transducer element temperature. Such transient temperature deviations are typically associated with an inability of the MR transducer element to read previously written data at the affected disk surface location, thereby rendering the stored information unrecoverable.
In the continuing effort to minimize head-to-disk clearance, manufacturers of disk drive systems recognize the importance of establishing a nominal head flyheight that is sufficient to avoid disk surface defects, such as asperities. A spacing tolerance is typically included within the nominal flyheight dimension for a given drive design in order to minimize the probability of contact between the slider/transducer and anticipated disk surface obstructions that may develop on the disk surface during and after manufacture.
Although the added spacing tolerance may indeed reduce the likelihood of head-to-disk contact, the resulting increase in head-to-disk spacing reduces the readback signal sensitivity of the transducer which impacts the degree to which the disk storage density may be increased. Further, adding a spacing tolerance to the flyheight specification across a family of common disk drives typically results in an unnecessarily large nominal flyheight for many of the common drives.
A number of screening approaches have been developed for use during disk drive manufacturing to identify heads that are flying with insufficient head-to-disk clearance. One such method for determining head-to-disk clearance is referred to as a Harmonic Ratio Flyheight (HRF) clearance test. The HRF test is a known method for measuring the flyheight of a slider/transducer assembly using a magnetic head-to-disk spacing signal. The HRF measurement method provides a continuous, instantaneous measurement of the ratio of two spectral lines in the spectrum of a readback signal. Both of the instantaneous spectral line amplitudes relate to the same volume element of the recording medium directly underneath the MR transducer. The HRF measurement method provides for the determination of the instantaneous head clearance with respect to the disk surface using a magnetic readback signal.
Although the HRF clearance test provides accurate head-to-disk spacing measurements, the HRF test method typically requires employment of a dedicated tester which may take several minutes to complete HRF testing of a disk drive. Additional complications arise if HRF measurements are to be made in the data zone rather than in the start/stop or landing zone. For example, a mechanical pusher may be required to position the heads at the outer disk diameter in the data zone, with HRF measurements being taken between two fixed motor speeds. These and other potential hardware and software complications associated with the HRF clearance test approach significantly reduce the attractiveness of implementing a fully autonomous in-situ HRF clearance testing capability within a direct access storage device.
There exists a keenly felt need in the data storage system manufacturing community for an apparatus and method for detecting low flying heads during disk drive manufacturing and, importantly, during subsequent use in the field. There exists a further need for an apparatus and method for detecting head-to-disk contact events so that surface defect locations may be identified and avoided. There exists yet a further need to provide such an apparatus and method which is suitable for incorporation into existing data storage systems, as well as into new system designs, and one that operates fully autonomously in-situ a data storage system. The present invention is directed to these and other needs.